The invention relates generally to permanent magnet structures, and in particular to new permanent magnet assemblies that do not require pole pieces restricting the aperture of the working region while producing compact solenoidal or longitudinal internal magnetic working fields.
Solenoidal and longitudinal magnetic fields are used extensively in the microwave tube and plasma physics communities in applications requiring the focusing and manipulation of charged particle trajectories. In the case of small microwave tubes, such as the kilowatt magnetrons used in microwave ovens, assembles of permanent magnets and high magnetic permeability pole pieces are commonly used to achieve the longitudinal field profiles required for operation of the device. Larger tubes, however, tend to employ the use of more massive wire solenoid coils or Helmholtz coil pairs in conjunction with bulky power supplies. The reliance on bulky coil-based magnets can be undesirable in cases where overall system compactness is a necessity as well as in cases with limited electrical power budgets.
By using magnetically rigid, high energy-product magnetic materials such as the rare earth magnets Neodymium-Iron-Boron or Samarium-Cobalt, it is feasible to build compact magnetic structures that generate a solenoidal or longitudinal magnetic field within the desired working region of the magnet assembly. Previous work by Tilak, et al., “Permanent Magnet Solenoids: A Catalog of Field Profiles,” ARL-TR-1123, Sept. 1996, as well as U. S. Pat. Nos. 5,126,713, 5,422,618, and 5,438,308 demonstrate examples of permanent magnet solenoids with a variety of working volume diameter to length aspect ratios. Unfortunately, in all of these configurations, the entrance apertures to the working region of the solenoid is either partially or completely blocked by the existence of magnetic pole pieces or cladding magnet arrays. This is most undesirable in cases requiring quick change-out of a microwave tube where one does not wish to disassemble or move the magnet or vice versa. This is also problematic for cases of devices with a constant or nearly constant cross-sectional area throughout in which magnetization is desired in only one finite section.
In the present invention, it was discovered that by properly choosing the shape and magnetization direction of permanent magnet assemblies, a solenoidal or longitudinally-directed magnetic field could be generated in the working region of a permanent magnet solenoid without the need for pole pieces or cladding magnet assemblies that completely or partially block the entrance aperture of the working region. The present invention fulfills the need for a compact method to generate a solenoidal or longitudinally-directed magnetic field in a given volume without the requirement of electrical power or magnetic pole pieces, and provides the ability to use an entrance aperture of the same cross-sectional area as that of the working volume of the magnet.